The present invention relates to a method for defining a photoresist of the type used in the manufacture of integrated circuits.
Integrated circuits are manufactured using photolithographic techniques which entail photoengraving and etching. In order to manufacture an integrated circuit, a number of photomasks are used to define different areas for diffusion, etching, oxidation, metallization, and other processes. Typically, the alignment of photomasks with the silicon wafer or substrate is done using a light having a wavelength which will not expose a photoresist layer which has been applied to the wafer. When the wafer and the photomask are in registration, the photoresist is exposed by another light having a wavelength which is shorter than the wavelength of the light used for alignment purposes. Typically, the light used for exposure is in the ultraviolet range, whereas the light used for alignment is in the visible range.
Since the photoresist layer has a thickness which is optically comparable to the wavelength of light used for exposure, standing waves occur in the photoresist layer. Such standing waves result in nonuniform exposure of the photoresist in that alternating layers parallel to the wafer surface are exposed and unexposed. As this is clearly undesirable, the manufacturers of photoresist materials have attempted to sensitize photoresists over a larger range of wavelengths in order to help to eliminate such layering. In that regard, positive photoresists are less prone to layering than are negative photoresists.